Thin film photovoltaic (PV) modules (also referred to as “solar panels”) based on cadmium telluride (CdTe) paired with cadmium sulfide (CdS) as the photo-reactive components are gaining wide acceptance and interest in the industry. CdTe is a semiconductor material having characteristics particularly suited for conversion of solar energy to electricity. For example, CdTe has an energy bandgap of about 1.45 eV, which enables it to convert more energy from the solar spectrum as compared to lower bandgap semiconductor materials historically used in solar cell applications (e.g., about 1.1 eV for silicon). Also, CdTe converts radiation energy in lower or diffuse light conditions as compared to the lower bandgap materials and, thus, has a longer effective conversion time over the course of a day or in cloudy conditions as compared to other conventional materials.
The junction of the n-type layer and the p-type layer is generally responsible for the generation of electric potential and electric current when the CdTe PV module is exposed to light energy, such as sunlight. Specifically, the cadmium telluride (CdTe) layer and the cadmium sulfide (CdS) form a p-n heterojunction, where the CdTe layer acts as a p-type layer (i.e., a positive, electron accepting layer) and the CdS layer acts as a n-type layer (i.e., a negative, electron donating layer). Free carrier pairs are created by light energy and then separated by the p-n heterojunction to produce an electrical current.
Reducing the thickness of the layers between the window glass and the cadmium telluride layer (e.g., the transparent conductive layer, the cadmium sulfide layer and any buffer layer(s) therebetween) can reduce the amount of absorption of radiation energy (e.g., solar energy) by the device prior to the energy reacting the cadmium telluride layer. Thus, the overall conversion efficiency of the device may be improved. Additionally, reducing the thickness of the cadmium sulfide layer can allow for more of the shorter wavelength (e.g., blue radiation) to reach the cadmium telluride layer, again improving the overall conversion efficiency of the device.
However, reducing the thickness of the cadmium sulfide layers, along with any other layers between the glass and the cadmium telluride layer, can create other problems that are potentially detrimental to the performance of the device. For example, a relatively thin cadmium sulfide layer can lead to interface defects such as pinholes that create localized junctions between the transparent conductive oxide layer and the cadmium telluride layer. Such defects can lower the open circuit voltage (Voc) of the device and reduce the fill factor of the device.
Thus, a need exists for cadmium telluride photovoltaic devices having improved energy conversion efficiency and/or device lifetime through reduced thicknesses of the layers between the window glass and the cadmium telluride layer, while reducing the side-effects typically associated with reduced thicknesses of those layers.